The typical application envisioned is the following: measurement of light transmission or reflection coefficient of a medium through which light passes, by means of a light-emitting cell and a photosensitive cell, or measurement of the variations of this transmission or reflection coefficient. In the presence of an ambient illumination of unknown and variable luminosity, or even an ambient illumination of much greater intensity than the illumination produced by the light-emitting cell, it will be understood that the measurement is not easy.
In order to overcome this drawback, a synchronous measurement is performed: the light-emitting cell emits periodic light pulses and the signal received by the photosensitive cell is observed during the emission times; in addition, a differential measurement is carried out at two different times, namely just before the start of the pulse and just before the end of the pulse. The difference in the measured signals will then represent an increase in illumination specifically due to the light produced by the light-emitting cell and having passed through the medium to be measured.
This differential measurement is called ‘correlated double sampling’ when the signals are measured by means of a sample-and-hold. A sample-and-hold is used notably when the signal coming from the measurement, obtained in analog form (this is the case for a photosensitive cell), has to be converted into a digital value by an analog-digital converter. In this case, the sample-and-hold is designed to directly perform a differential measurement, in other words it directly delivers an electrical voltage value representing, rather than each measurement of illumination, the difference between the measurements made just before the start of the pulse and just before the end of the pulse.
FIG. 1 shows an example of schematic circuit diagram of a sample-and-hold capable of making this direct differential measurement. The input E receives the measurement signal Ve (for example a signal produced by a photosensitive cell) which is equal to a reference level Vr, representing the ambient illumination, which is unknown, that exists in the absence of the light pulses. The input E is connected to a first terminal of an input capacitor Ce1 whose second terminal may be connected either to ground via a switch K1 or to the inverting input (−) of an operational amplifier AMP via a switch K′1. The operational amplifier, with high gain and high input impedance, has its noninverting input (+) connected to ground and its two inputs are considered as being virtually at ground potential. An integrating capacitor Cs1 is connected between the inverting input and the output S of the operational amplifier. A switch K″1 allows the integrating capacitor Cs1 to be short-circuited in order to periodically discharge it.
The switches K1 and K″1 are closed at the same time as the switch K′1 is open, during a reset phase H1 that ends just before the light pulse emitted by the light-emitting cell. The input capacitor Ce1 therefore charges up to the reference voltage Ve=Vr present at the input just before the light pulse; the integrating capacitor Cs1 is completely discharged. Then, the switches K1 and K″1 are opened and the switch K′1 is closed during a sampling phase H2 following the reset phase. During this phase H2, the light pulse is emitted. The sampling phase ends before the end of the light pulse, but only when the signal from the photoelectric cell is stabilized. The voltage Ve at the input E is then Vm, which is different from Vr due to the light pulse.
Owing to the conservation of the charges on the joined and isolated electrodes of the capacitors Ce1 and Cs1 (total charge Ce1Vr), the charges are divided up between the capacitor Ce1 and the capacitor Cs1 and, owing to the potential of the inverting input of the amplifier being virtually held at ground, the potential adopted by the output S is Vs=−Ce1(Vm−Vr)/Cs1.
This output potential Vs is therefore equal to the inverse of Vm−Vr if Ce1 and Cs1 are equal, or proportional to −(Vm−Vr) if this is not the case.
The output potential Vs is subsequently maintained during a hold phase (after the end of the sampling period H2, when all the switches K1, K′1, K″1 are open); this potential, held at its value until the next period H1, can be used for an analog-digital conversion since it is proportional to Vm−Vr; it directly represents the signal difference due to the light pulse, irrespective of the level of ambient light.
FIG. 2 shows the signal timing diagram corresponding to the operation of the circuit in FIG. 1:                the line a represents the periodic light pulse emission time intervals;        the line b represents the signal detected by the photosensitive cell; the signal level due to the ambient light just before the light pulse is Ve=Vr; the signal Vm measured at the end of the light pulse represents the addition of the ambient illumination and the illumination due to the light pulse;        the line c represents the control period of the switches K1 and K″1, corresponding to the reset phase H1; its duration is, for example, the same as that of the light pulse but beginning at a time T0 before the start of the light pulse and ending at a time T1 just before the end of the light pulse;        the line d represents the sampling signal H2 which controls the switch K′1; it begins just after the end of the reset signal H1 and ends at a time T2 just before the end of the light pulse; its duration is that of the light pulse;        finally, the line e represents the inverse −Vs of the output voltage Vs, which goes to zero during the period H1 (reset) and which takes the value Vm−Vr during the period H2 (sampling) then keeps this value until the following measurement (hold).        
This method of correlated double sampling associated with a synchronous detection yields excellent results.
Other types of samplers, more or less complex, exist and are capable of performing the correlated double sampling operation. FIG. 3 shows a particularly simple sampler from the prior art; FIG. 4 shows a differential sample-and-hold, accomplishing the same correlated double sampling function, but on a differential useful signal Ve representing the difference between two voltages Vep and Ven referenced with respect to ground.
The invention is based on the observation that, in some applications, this measurement method by synchronous detection and correlated double sampling is not satisfactory. These applications are those in which the reference level of the measured signal (for example the level Vr representing the ambient illumination) is not sufficiently stable. In fact, the method assumes that the reference level has not changed between the beginning and the end of the signal H2.
However, the duration of the signal H2 cannot be reduced below a minimum time. It must be at least long enough so that the measurement signal has had the time to stabilize after the start of the light pulse. The stabilization time is linked to the response time of the photosensitive cell and of all the electronic circuits that allow the voltage Ve to be established at the input of the sample-and-hold.
Typically, the duration of H2 can be 30 microseconds. But if the reference level Vr has the time to vary significantly during this period, the differential measurement will be affected by an error since it takes the difference between the level Vm attained at time T2 during the pulse and the reference level V1 considered at time T1 before the start of the pulse, rather than taking the difference at the same moment.
In a particularly important application envisioned here, it has been noticed that the variations of the ambient illumination over the duration of the short pulse of the synchronous measurement could lead to a significant measurement error, rendering the application inoperative in certain cases. This application is the detection of rain on a vehicle windshield with a view to triggering the automatic sweeping by the windshield wipers. The detection is carried out by a photoelectric cell (visible or infrared light) which detects the changes in reflection coefficient of the windshield depending on the presence or absence of rain. The change in reflection induces small changes in measurement signal level, where these changes may be a thousand times weaker than the signal due to the ambient illumination. However, the ambient illumination is subject to variations that are all the more rapid the higher the vehicle speed (passage under bridges, trees, etc.). The variations in 30 microseconds are very significant, and induce measurement errors that can mask the variations in reflection coefficient that it is desired to measure. The presence of rain could be detected whereas this is simply the result of a measurement error.